A large number and variety of jet engine exhaust nozzles have been provided which are concerned with the reduction or attenuation of the noise produced when the engine jet issues from the exhaust nozzle exit. In one type of jet noise suppressor, the nozzle exit is configured so that the ratio of jet periphery to jet cross sectional area is much larger than for a circle, the increased jet surface promoting rapid mixing of the jet with the surrounding air and thereby shortening the length of the mixing region, which is a source of noise. Jet noise suppressors of this type may be multilobed as a Greatrex nozzle, multitube nozzles, or slit nozzles.
A further type of jet noise suppressor involves acoustical treatment of the inside of the engine tail pipe and exhaust nozzle. This acoustical treatment involves locating sound absorbing material within the tail pipe and nozzle in regions where the flow velocity of the gas is relatively low so as to minimize the thrust loss due to fluid friction between the flowing gas and the acoustical material.
Another type of jet noise suppressing nozzle incorporates a more or less cylindrical shroud which surrounds the jet exit. The diameter of the shroud is greater than that of the jet exit so that external air flow may enter the open annulus between the jet and the leading edge of the shroud. The length of the shroud is typically about two jet diameters and the shroud forms a barrier surrounding the region of intense noise caused by mixing of the jet with the air induced into the shroud, and reduces radiated noise by acting as a barrier to radiation. The gas exiting from the shroud presents reduced noise problems since downstream of the shroud the jet velocity is reduced by mixing and thus further noise generation is accordingly reduced.
Each of the above proposals suffers disadvantages. In general, the most serious disadvantage of jet noise suppressors of the prior art is that such devices achieve a reduction in jet noise at the expense or cost of a related loss of thrust. An estimate of thrust loss for a typical installation is approximately one percent of nozzle ideal isentropic gross thrust for each decibel of noise reduction when compared with the noise and thrust performance of a conventional convergent or convergent-divergent exhaust nozzle.
More particularly, jet noise suppressors which depend on increasing the ratio of jet periphery to cross sectional area, such as lobe, multitube and slit types, inherently incorporate, within the nozzle, a relatively large surface in contact with the flowing exhaust gases and thereby incur excessive fluid friction losses. Such prior art suppressors generally include a large effective base or boattail area which cause excessive aerodynamic drag, and, therefore, an excessive loss of thrust. It will be appreciated that the shrouded jet type of noise suppressor referred to above produces very high drag except at low speeds, i. e., at speeds less than a Mach number of 0.30.
The disadvantages of what has been referred to as acoustical treatment type noise suppressors are of a different kind. In particular, the suppressors which depend on acoustical treatment of the inside of the tail pipe and nozzle produce minimum thrust loss. Firstly, there is a loss due to the fluid friction between the flowing exhaust gas and the acoustical material although this loss is relatively small. Further, and more importantly, this type of jet noise suppressor does not attenuate the noise generated downstream of the jet exit by the mixing of the jet with external air and hence fails to quell a critical source of jet noise.
A further disadvantage common to most jet noise suppressors of the prior art is that no deliberate attempt is made to break up the wave fronts of sound generated within the engine and tail pipe as each sound wave issues from the jet exit or exits. When, as for many prior art jet noise suppressors, the nozzle exits are more or less coplanar, it is noted that a sound wave front, normal to the nozzle axis of symmetry and moving toward the nozzle exit(s), will issue from all the nozzle exits simultaneously and recombine as a single strong pressure wave downstream of the exits. The aggregation of such waves of various frequencies constitute noise. It is pointed out that the character of the noise generated within the engine and tail pipe is not appreciably altered by systems employing a multiplicity of coplanar jet exits as the sound waves issue from the exhaust system into the surrounding atmosphere.
An additional disadvantage of prior art jet noise suppressors is that they are configured such that kinetic energy of the jet downstream of the jet exit or exits cannot be utilized to reduce drag. One exception to this statement is the shroud type noise suppressor referred to above which, at very low flight speeds, may utilize kinetic energy of the jet to provide some thrust augmentation. However, at speeds above about Mach No. 0.30, this thrust augmentation vanishes and at higher speeds the drag produced by the shroud becomes undesirably great.
The following patents disclose jet noise suppressors or silencers of general interest although this list is not, of course, represented as being in any way complete or exhaustive: U.S. Pat. No. 2,865,169 (Hausmann); U.S. Pat. No. 2,952,124 (Pearson); U.S. Pat. No. 3,069,048 (Griffith); U.S. Pat. No. 3,187,501 (Quick); and U.S. Pat. No. 3,463,402 (Langston, Jr.). Generally speaking, the jet noise suppressors disclosed in these patents suffer one or more of the disadvantages discussed above.